1 Answer
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You can try whether sudo -s still works (which would allow you to add yourself to the admin group again), but probably it won't.

Do the following

Reboot into Single User mode by keeping Cmd-S pressed during reboot

/sbin/mount -uw /

rm /var/db/.AppleSetupDone

reboot

This will trick your Mac into behaving like a new installation (but with your data still intact). Follow the setup without migrating any data etc., near the end you will have the option to create a new user with admin privileges. Make sure to pick a new username here to avoid any conflicts.